RMS (Root-Mean Square)
It is the minimum continuous sinewave power output measured in watts that can be delivered by the amplifier into a rated load, over the specified bandwidth with maximum specified total harmonic distortion.
PMPO (Peak Music Power Output)
This is the capability of an amplifier either over a short duration during which there is no perceptible sag of the power supplies during the measurement, or when very high-quality power supplies are used with normal music input. It appears that some companies simply multiply RMS output by 8 or (2√2)2 . RMS power is a more relied upon method for comparison.
Even though PMPO is not a well-defined standard of measurement, it is often seen on the product packaging or on the panel of a car audio amplifier (Fig. 1). Different manufacturers use different definitions for this rating. PMPO is more of a marketing tool to attract customers who are looking for a higher power rating with lower cost audio amplifiers.